A method of metal container manufacture in current use by the metal container industry is the two-piece can process. This process involves forming a drawn cup from a metal sheet and then deep drawing the cup into a can configuration. After the can body is completely configurated and decorated, but before the end is assembled onto the body, the interior surface of the can body is coated with a protective coating of a synthetic resin material.
It is conventional practice to apply, as the coating material, a heat hardenable resin dispersed in an aqueous medium which is sprayed into the interior walls of the container. The open-ended can is caused to be passed through an internal coating station, normally a coating "tunnel," where the coating liquid is sprayed into the interior of the can to coat its internal surface. The wet coated container is then passed through an oven in which hot air is circulated to evaporate the aqueous medium and harden the coating.
In some conventional systems, the coating is sprayed into the interior of the container while inverted in an upright position on a reticulated belt. In such systems, as the containers travel through the coating tunnel, the interior surface walls of the containers are contacted with an aqueous dispersion of a coating resin by spraying means, usually comprising a plurality of single orifice nozzles disposed under and along the reticulated belt, which direct a sequence of wide atomized sprays of wet resin coating onto the interior walls.
Substantially all the conventional nozzles used in such applications are of the single orifice type which make controlling the application of the coating to the interior surface difficult, especially near the open end of the container.
The most commonly used airless nozzle includes an internal, hemispherical passage termination which is cut through by an external, V-shaped groove to form an elongated, elliptical-like orifice. Liquid material pumped at high pressures through such a spray nozzle is forced by the hemispherical termination of the passageway to converge in its flow at and through the elongated orifice. Because of the converging flow at the orifice, the liquid material is expelled through the orifice into a planar, expanding, fan-like film which breaks into spray particles which are carried by their momentum to the article target.
Such prior systems are plagued by excessive overspray and nozzle misting, each of which is costly due to the wasted coating resin. In addition, it is desirable to place more coating near the top and the bottom of the can and less in the middle sidewall of the container as the top of the container is worked upon during the necking operation and the bottom of the container is in contact with the pin tip of the pin chain. The middle sidewall normally has minimal contact and needs less coating.
Thus, there has developed in the metallic container manufacturing industry a need for an internal coating nozzle assembly capable of placing the coating where it is most required within the interior of the container while also minimizing the amount of wasted coating liquid normally produced by the operation.